An automotive automatic power transmission uses, in the transmission mechanism thereof, a plurality of fluid operated frictional units which are selectively actuated by fluid pressure so as to produce any of a plurality of gear ratios between a driving member connected to the engine and a driven member connected to the final drive unit of the automotive driveline.
The fluid pressure to actuate these fluid operated frictional units is developed by a pressure regulator valve incorporated into the hydraulic control system of the transmission. The pressure regulator valve of an ordinary hydraulic transmission control system comprises a valve spool formed with a pressure acting area to be acted upon by a fluid pressure delivered from a hydraulic pump driven by the engine. The force exerted on the valve spool by the fluid pressure thus applied to the pressure acting area of the valve spool is opposed by a suitable spring constantly urging the valve spool to move against the force resulting from the fluid pressure so that an excess of fluid pressure higher than a predetermined value is discharged from the pressure regulator valve. The pressure regulator valve is thus operable for producing a line pressure dictated by the force of the spring and the pressure acting area formed on the valve spool.
In a modified form of pressure regulator valve used in a known hydraulic transmission control system, the valve spool is formed with another pressure acting area to be acted upon by a throttle pressure which is variable with the load on the engine or, more specifically, the degree to which the throttle valve provided in the carburetor of the engine is open. The throttle pressure is applied to the valve spool of the pressure regulator valve in such a manner as to reinforce the force of the spring so that the line pressure acting on the first pressure acting area is opposed by not only the force of the spring but the force resulting from the throttle pressure acting on the second pressure acting area of the valve spool. The line pressure developed by the pressure regulator valve of this nature is variable with the throttle pressure and accordingly the opening degree of the carburetor throttle valve.
The throttle pressure developed in response to the opening degree of a carburetor throttle valve tends to be excessively high when the opening degree of the carburetor throttle valve is greater than a certain value and, for this reason, does not accurately reflect the torque output characteristics of the engine operating under conditions in which the carburetor throttle valve is open to such degrees. The line pressure produced on the basis of such a throttle pressure is inappropriate for achieving optimum shift points in the hydraulic control system and actuating the fluid operated frictional units of the transmission mechanism without creating shocks during shifting between gear ratios. This impairs the gear shifting performance of the power transmission and will discomfort the occupants of a vehicle during shifting between gear ratios.
Such a problem can be solved if means are provided to limit the throttle pressure below a predetermined value corresponding to a certain opening degree of a carburetor throttle valve so that the resultant throttle pressure approximates the torque output of the engine. In this instance, however, a problem arises in that the line pressure developed on the basis of such a throttle pressure is deficient, especially under low to part throttle conditions of the engine, for those frictional units of the transmission mechanism which are to be put into operation when a forward drive gear position operable for effecting engine braking in the vehicle is selected. If the line pressure actuating the frictional units contributing to the establishment of the particular gear position is deficient for enabling the frictional units to provide adequate torque transmission efficiencies, the torque output of the engine can not be reacted upon efficiently by the power train constituting the transmission mechanism with the result that engine braking can not be effected reliably.
With a view to providing a solution to this problem, it has thus far been proposed and put into practice to have the transmission throttle valve of a hydraulic transmission control system combined with a throttle back-up valve operative to boost the throttle pressure under conditions in which the engine braking effect is available or with a valve adapted to produce a constant fluid pressure higher than the throttle pressure and deliver the constant fluid pressure to the pressure regulator valve in lieu of the throttle valve under such conditions. Incorporation of such an additional valve however adds to the complexity of the whole construction of the hydraulic control system and is therefore now fully acceptable.
Another expedient to solve the above described problem is to provide in a hydraulic control system such means that are adapted to feed the line pressure in lieu of the throttle pressure under the conditions in which engine braking is operable. In this instance, the valve spool of the pressure regulator valve is subjected to the line pressure on both of the previously mentioned first and second pressure acting areas thereof and, as a consequence, the line pressure applied to the valve spool is effective only on a difference between the two pressure acting areas which are acted upon by the line pressure in the opposite directions. The valve spool is therefore made more responsive to the fluctuations in the pump pressure delivered to the pressure regulator valve and causes the line pressure to fluctuate during engine braking.
The present invention contemplates a line pressure control device useful for overcoming all these problems that have been encountered in known hydraulic control systems of automatic power transmissions for automotive vehicles.